LeetCode 42. Trapping Rain Water

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class Solution {
public:
    int trap(vector<int>& height) {
        int n = height.size();
        if (n == 0) return 0;
        vector<int> leftMax(n);
        leftMax[0] = height[0];
        for (int i = 1; i < n; ++i)
            leftMax[i] = max(leftMax[i - 1], height[i]);
        vector<int> rightMax(n);
        rightMax[n - 1] = height[n - 1];
        for (int i = n - 2; i >= 0; --i)
            rightMax[i] = max(rightMax[i + 1], height[i]);
        int ans = 0;
        for (int i = 0; i < n; ++i)
            ans += min(leftMax[i], rightMax[i]) - height[i];
        return ans;
    }
};
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class Solution {
public:
    int trap(vector<int>& height) {
        int n = height.size();
        stack<int> stk;
        int ans = 0;
        for (int i = 0; i < n; ++i) {
            while (!stk.empty() && height[i] > height[stk.top()]) {
                int top = stk.top(); stk.pop();
                if (stk.empty()) break;
                int left = stk.top();
                int currWidth = i - left - 1;
                int currHeight = min(height[i], height[left]) - height[top];
                ans += currWidth * currHeight;
            }
            stk.push(i);
        }
        return ans;
    }
};
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class Solution {
public:
    int trap(vector<int>& height) {
        int n =  height.size();
        if (n < 3) return 0;
        int left = 0, right = n - 1, leftMax = 0, rightMax = 0;
        int ans = 0;
        while (left < right) {
            if (height[left] < height[right]) {
                if (height[left] < leftMax)
                    ans += leftMax - height[left];
                else
                    leftMax = height[left];
                ++left;
            } else {
                if (height[right] < rightMax)
                    ans += rightMax - height[right];
                else
                    rightMax = height[right];
                --right;
            }
        }
        return ans;
    }
};